1. Field of the Invention
The present invention relates to an air data sensor that measures an airspeed, an angle of attack, a sideslip angle, and turbulence in an airborne device that measures a wind velocity in a distant region at a distance of several tens of meters to about 1 km on the basis of a Doppler effect by emitting a laser light into the atmosphere and receiving a scattered light produced by scattering of the laser light in the atmosphere.
2. Description of the Related Art
In terms of flight safety and operational efficiency, the airspeed is the most important parameter for an aircraft flying in the atmosphere, and almost all of the presently used aircrafts use a Pitot tube as an airspeed measuring means. In the Pitot tube, the total pressure and static pressure of the air are measured and the airspeed is determined from a dynamic pressure, which is the difference therebetween. The airflow direction is measured by an vane. However, since the dynamic pressure measured by the Pitot tube is proportional to the second power of airspeed, the measurement error at a low velocity is high and the Pitot tube is unsuitable for velocity measurements in a low-speed range. An airspeed region in which the Pitot tube can be used is usually equal to or higher than 30 m/s. When the velocity is below this range or the airflow direction is significantly different from the axial line of the Pitot tube, the velocity measurements are inherently impossible. Further, since the vane for measuring the airflow direction is a movable component, vibrations and decrease in responsiveness caused by the mass of the vane become a problem. Therefore, in a typical aircraft having a Pitot tube as an airspeed sensor, values of airspeed in a low-speed range have a large measurement error or measurements cannot be conducted. The inventors have previously suggested an ultrasonic air data sensor (see Japanese Patent Application Laid-open No. 2004-264184 “Ultrasonic Air Data Sensor”, published on Sep. 24, 2004).
However, in a sensor that is directly mounted on the fuselage as a Pitot tube or an ultrasonic air data sensor (see Hamaki Inokuchi, National Aerospace Laboratory Materials TM-776, “Position Error for the Airspeed Sensor of the Multi-Purpose Aviation Laboratory MuPAL-α”, August 2003), the fuselage itself affects the flow field, thereby causing a measurement error called a position error. Therefore, a large number of calibration flight tests are necessary when the sensor is used. In general, this test is conducted repeatedly early in the morning in a time interval with calm air at a low altitude in two directions. As a result, problems are associated with noise. Moreover, since the test is easily affected by weather, a large number of test days are required. Moreover, since a calibration flight test is not conducted in a sideslip flight or close to a stalling velocity, measurement accuracy of airspeed under special flight conditions cannot be ensured.
A Doppler LIDAR using a laser light has been researched and developed as a device that measures an airflow at a distance sufficient to prevent the aircraft from affecting the flow field (see, for example, H. Inokuchi, H. Tanaka, and T. Ando, “Development of an Onboard Doppler LIDAR for Flight Safety,” Vol. 46, No. 4 of the Journal of Aircraft, AIAA, July-August 2009). LIDAR is an abbreviation for “Light Detection And Ranging”, that is, a technique for detection that uses light. With this technique, an emitted light is scattered by fine aerosol floating in the atmosphere, the scattered light is received, and the frequency variation (wavelength variation) according to the Doppler effect is measured, whereby the wind velocity is measured. When a Doppler LIDAR is used for turbulence avoidance flights of aircrafts, the effective measurement distance of at least 10 km is required and a laser light source with a correspondingly high output is needed. However, since the distance at which the aircraft itself produces no effect on the flow field is several tens of meters, a device with a comparatively low output is suitable for practical use as an air data sensor.
When a Doppler shift amount is measured, the frequency measurement range is usually limited by the restrictions placed by the capabilities of an AD converter. Therefore, when an airflow with a velocity higher than the wind velocity is superimposed, as in the case of an aircraft, the frequency measurement range is offset by inputting a reference velocity from the outside. Therefore, with the conventional technology, the reference velocity should be inputted from the outside and the Doppler LIDAR has no function of determining autonomously the absolute airspeed.
As mentioned hereinabove, the airspeed is extremely important for an aircraft and therefore a system including multiple Pitot tubes is usually used. However, if by any chance a malfunction occurs, it will surely produce a devastating operational effect. Even with the system including multiple Pitot tubes, since all of them operate on the same principle, the calibration also has to be performed with the same tester. Therefore, when airspeed sensors of completely different types are used, a significant increase in redundancy can be expected.